Persistent phenotypic shift in cardiac fibroblasts: impact of transient renin angiotensin system inhibition

Ramipril After Transcatheter Aortic Valve Implantation in Patients Without Reduced Ejection Fraction: The RASTAVI Randomized Clinical Trial

BACKGROUND

Patients with aortic stenosis may continue to have an increased risk of heart failure, arrhythmias, and death after successful transcatheter aortic valve implantation. Renin-angiotensin system inhibitors may be beneficial in this setting. We aimed to explore whether ramipril improves the outcomes of patients with aortic stenosis after transcatheter aortic valve implantation.

METHODS AND RESULTS

PROBE (Prospective Randomized Open, Blinded Endpoint) was a multicenter trial comparing ramipril with standard care (control) following successful transcatheter aortic valve implantation in patients with left ventricular ejection fraction >40%. The primary end point was the composite of cardiac mortality, heart failure readmission, and stroke at 1-year follow-up. Secondary end points included left ventricular remodeling and fibrosis. A total of 186 patients with median age 83 years (range 79-86), 58.1% women, and EuroSCORE-II 3.75% (range 3.08-4.97) were randomized to receive either ramipril (n=94) or standard treatment (n=92). There were no significant baseline, procedural, or in-hospital differences. The primary end point occurred in 10.6% in the ramipril group versus 12% in the control group (P=0.776), with no differences in cardiac mortality (ramipril 1.1% versus control group 2.2%, P=0.619) but lower rate of heart failure readmissions in the ramipril group (3.2% versus 10.9%, P=0.040). Cardiac magnetic resonance analysis demonstrated better remodeling in the ramipril compared with the control group, with greater reduction in end-systolic and end-diastolic left ventricular volumes, but nonsignificant differences were found in the percentage of myocardial fibrosis.

CONCLUSIONS

Ramipril administration after transcatheter aortic valve implantation in patients with preserved left ventricular function did not meet the primary end point but was associated with a reduction in heart failure re-admissions at 1-year follow-up.

REGISTRATION

URL: https://www.clinicaltrials.gov; Unique Identifier: NCT03201185.

Fibroblasts under pressure: cardiac fibroblast responses to hypertension and antihypertensive therapies

Approximately 50% of Americans have hypertension, which significantly increases the risk of heart failure. In response to increased peripheral resistance in hypertension, intensified mechanical stretch in the myocardium induces cardiomyocyte hypertrophy and fibroblast activation to withstand increased pressure overload. This changes the structure and function of the heart, leading to pathological cardiac remodeling and eventual progression to heart failure. In the presence of hypertensive stimuli, cardiac fibroblasts activate and differentiate to myofibroblast phenotype capable of enhanced extracellular matrix secretion in coordination with other cell types, mainly cardiomyocytes. Both systemic and local renin-angiotensin-aldosterone system activation lead to increased angiotensin II stimulation of fibroblasts. Angiotensin II directly activates fibrotic signaling such as transforming growth factor β/SMAD and mitogen-activated protein kinase (MAPK) signaling to produce extracellular matrix comprised of collagens and matricellular proteins. With the advent of single-cell RNA sequencing techniques, heterogeneity in fibroblast populations has been identified in the left ventricle in models of hypertension and pressure overload. The various clusters of fibroblasts reveal a range of phenotypes and activation states. Select antihypertensive therapies have been shown to be effective in limiting fibrosis, with some having direct actions on cardiac fibroblasts. The present review focuses on the fibroblast-specific changes that occur in response to hypertension and pressure overload, the knowledge gained from single-cell analyses, and the effect of antihypertensive therapies. Understanding the dynamics of hypertensive fibroblast populations and their similarities and differences by sex is crucial for the advent of new targets and personalized medicine.

Myofibroblast specific targeting approaches to improve fibrosis treatment

Fibrosis has been shown to develop in individuals with underlying health conditions, especially chronic inflammatory diseases. Fibrosis is often diagnosed in various organs, including the liver, lungs, kidneys, heart, and skin, and has been described as excessive accumulation of extracellular matrix that can affect specific organs in the body or systemically throughout the body. Fibrosis as a chronic condition can result in organ failure and result in death of the individual. Understanding and identification of specific biomarkers associated with fibrosis has emerging potential in the development of diagnosis and targeting treatment modalities. Therefore, in this review, we will discuss multiple signaling pathways such as TGF-β, collagen, angiotensin, and cadherin and outline the chemical nature of the different signaling pathways involved in fibrogenesis as well as the mechanisms. Although it has been well established that TGF-β is the main catalyst initiating and driving multiple pathways for fibrosis, targeting TGF-β can be challenging as this molecule regulates essential functions throughout the body that help to keep the body in homeostasis. We also discuss collagen, angiotensin, and cadherins and their role in fibrosis. We comprehensively discuss the various delivery systems used to target collagen, angiotensin, and cadherins to manage fibrosis. Nevertheless, understanding the steps by which this molecule drives fibrosis development can aid in the development of specific targets of its cascading mechanism. Throughout the review, we will demonstrate the mechanism of fibrosis targeting to improve targeting delivery and therapy.

Insights Into Vascular Anomalies, Cancer, and Fibroproliferative Conditions: The Role of Stem Cells and the Renin-Angiotensin System

Cells exhibiting embryonic stem cell (ESC) characteristics have been demonstrated in vascular anomalies (VAs), cancer, and fibroproliferative conditions, which are commonly managed by plastic surgeons and remain largely unsolved. The efficacy of the mTOR inhibitor sirolimus, and targeted therapies that block the Ras/BRAF/MEK/ERK1/2 and PI3KCA/AKT/mTOR pathways in many types of cancer and VAs, further supports the critical role of ESC-like cells in the pathogenesis of these conditions. ESC-like cells in VAs, cancer, and fibroproliferative conditions express components of the renin-angiotensin system (RAS) – a homeostatic endocrine signaling cascade that regulates cells with ESC characteristics. ESC-like cells are influenced by the Ras/BRAF/MEK/ERK1/2 and PI3KCA/AKT/mTOR pathways, which directly regulate cellular proliferation and stemness, and interact with the RAS at multiple points. Gain-of-function mutations affecting these pathways have been identified in many types of cancer and VAs, that have been treated with targeted therapies with some success. In cancer, the RAS promotes tumor progression, treatment resistance, recurrence, and metastasis. The RAS modulates cellular invasion, migration, proliferation, and angiogenesis. It also indirectly regulates ESC-like cells via its direct influence on the tissue microenvironment and by its interaction with the immune system. In vitro studies show that RAS inhibition suppresses the hallmarks of cancer in different experimental models. Numerous epidemiological studies show a reduced incidence of cancer and improved survival outcomes in patients taking RAS inhibitors, although some studies have shown no such effect. The discovery of ESC-like cells that express RAS components in infantile hemangioma (IH) underscores the paradigm shift in the understanding of its programmed biologic behavior and accelerated involution induced by β-blockers and angiotensin-converting enzyme inhibitors. The findings of SOX18 inhibition by R-propranolol suggests the possibility of targeting ESC-like cells in IH without β-adrenergic blockade, and its associated side effects. This article provides an overview of the current knowledge of ESC-like cells and the RAS in VAs, cancer, and fibroproliferative conditions. It also highlights new lines of research and potential novel therapeutic approaches for these unsolved problems in plastic surgery, by targeting the ESC-like cells through manipulation of the RAS, its bypass loops and converging signaling pathways using existing low-cost, commonly available, and safe oral medications.

Advanced In Vitro Modeling to Study the Paradox of Mechanically Induced Cardiac Fibrosis

In heart failure, cardiac fibrosis is the result of an adverse remodeling process. Collagen is continuously synthesized in the myocardium in an ongoing attempt of the heart to repair itself. The resulting collagen depositions act counterproductively, causing diastolic dysfunction and disturbing electrical conduction. Efforts to treat cardiac fibrosis specifically have not been successful and the molecular etiology is only partially understood. The differentiation of quiescent cardiac fibroblasts to extracellular matrix-depositing myofibroblasts is a hallmark of cardiac fibrosis and a key aspect of the adverse remodeling process. This conversion is induced by a complex interplay of biochemical signals and mechanical stimuli. Tissue-engineered 3D models to study cardiac fibroblast behavior in vitro indicate that cyclic strain can activate a myofibroblast phenotype. This raises the question how fibroblast quiescence is maintained in the healthy myocardium, despite continuous stimulation of ultimately profibrotic mechanotransductive pathways. In this review, we will discuss the convergence of biochemical and mechanical differentiation signals of myofibroblasts, and hypothesize how these affect this paradoxical quiescence. Impact statement Mechanotransduction pathways of cardiac fibroblasts seem to ultimately be profibrotic in nature, but in healthy human myocardium, cardiac fibroblasts remain quiescent, despite continuous mechanical stimulation. We propose three hypotheses that could explain this paradoxical state of affairs. Furthermore, we provide suggestions for future research, which should lead to a better understanding of fibroblast quiescence and activation, and ultimately to new strategies for the prevention and treatment of cardiac fibrosis and heart failure.

RAS inhibition in resident fibroblast biology

Angiotensin II (Ang II) is a primary mediator of profibrotic signaling in the heart and more specifically, the cardiac fibroblast. Ang II-mediated cardiomyocyte hypertrophy in combination with cardiac fibroblast proliferation, activation, and extracellular matrix production compromise cardiac function and increase mortality in humans. Profibrotic actions of Ang II are mediated by increasing production of fibrogenic mediators (e.g. transforming growth factor beta, scleraxis, osteopontin, and periostin), recruitment of immune cells, and via increased reactive oxygen species generation. Drugs that inhibit Ang II production or action, collectively referred to as renin angiotensin system (RAS) inhibitors, are first line therapeutics for heart failure. Moreover, transient RAS inhibition has been found to persistently alter hypertensive cardiac fibroblast responses to injury providing a useful tool to identify novel therapeutic targets. This review summarizes the profibrotic actions of Ang II and the known impact of RAS inhibition on cardiac fibroblast phenotype and cardiac remodeling.

COVID-19, Renin-Angiotensin System and Endothelial Dysfunction

The newly emergent novel coronavirus disease 2019 (COVID-19) outbreak, which is caused by SARS-CoV-2 virus, has posed a serious threat to global public health and caused worldwide social and economic breakdown. Angiotensin-converting enzyme 2 (ACE2) is expressed in human vascular endothelium, respiratory epithelium, and other cell types, and is thought to be a primary mechanism of SARS-CoV-2 entry and infection. In physiological condition, ACE2 via its carboxypeptidase activity generates angiotensin fragments (Ang 1-9 and Ang 1-7), and plays an essential role in the renin-angiotensin system (RAS), which is a critical regulator of cardiovascular homeostasis. SARS-CoV-2 via its surface spike glycoprotein interacts with ACE2 and invades the host cells. Once inside the host cells, SARS-CoV-2 induces acute respiratory distress syndrome (ARDS), stimulates immune response (i.e., cytokine storm) and vascular damage. SARS-CoV-2 induced endothelial cell injury could exacerbate endothelial dysfunction, which is a hallmark of aging, hypertension, and obesity, leading to further complications. The pathophysiology of endothelial dysfunction and injury offers insights into COVID-19 associated mortality. Here we reviewed the molecular basis of SARS-CoV-2 infection, the roles of ACE2, RAS signaling, and a possible link between the pre-existing endothelial dysfunction and SARS-CoV-2 induced endothelial injury in COVID-19 associated mortality. We also surveyed the roles of cell adhesion molecules (CAMs), including CD209L/L-SIGN and CD209/DC-SIGN in SARS-CoV-2 infection and other related viruses. Understanding the molecular mechanisms of infection, the vascular damage caused by SARS-CoV-2 and pathways involved in the regulation of endothelial dysfunction could lead to new therapeutic strategies against COVID-19.

Renin-Angiotensin System Inhibition Following Transcatheter Aortic Valve Replacement

BACKGROUND

Several studies have demonstrated the benefits of transcatheter aortic valve replacement (TAVR) in patients with aortic stenosis, but the presence of persistent fibrosis and myocardial hypertrophy has been related to worse prognosis.

OBJECTIVES

The aim of this study was to explore the potential benefits of renin-angiotensin system (RAS) inhibitors on left ventricular remodeling and major clinical outcomes following successful transcatheter aortic valve replacement (TAVR).

METHODS

Patients from 10 institutions with severe aortic stenosis who underwent TAVR between August 2007 and August 2017 were included. All baseline data were prospectively recorded, and pre-specified follow-up was performed. Doses and types of RAS inhibitors at discharge were recorded, and matched comparison according to their prescription at discharge was performed.

RESULTS

A total of 2,785 patients were included. Patients treated with RAS inhibitors (n = 1,622) presented similar surgical risk scores but a higher rate of all cardiovascular risk factors, coronary disease, and myocardial infarction. After adjustment for these baseline differences, reduction of left ventricular volumes and hypertrophy was greater and cardiovascular mortality at 3-year follow-up was lower (odds ratio: 0.59; 95% confidence interval: 0.41 to 0.87; p = 0.007) in patients treated with RAS inhibitors. Moreover, RAS inhibitors demonstrated a global cardiovascular protective effect with significantly lower rates of new-onset atrial fibrillation, cerebrovascular events, and readmissions.

CONCLUSIONS

Post-TAVR RAS inhibitors are associated with lower cardiac mortality at 3-year follow-up and offer a global cardiovascular protective effect that might be partially explained by a positive left ventricular remodeling. An ongoing randomized trial will help confirm these hypothesis-generating findings. (Renin-Angiotensin System Blockade Benefits in Clinical Evolution and Ventricular Remodeling After Transcatheter Aortic Valve Implantation [RASTAVI]; NCT03201185).

Matrix stiffness controls cardiac fibroblast activation through regulating YAP via AT1 R

Cardiac fibrosis is a common pathway leading to heart failure and involves continued activation of cardiac fibroblasts (CFs) into myofibroblasts during myocardium damage, causing excessive deposition of the extracellular matrix (ECM) and thus increases matrix stiffness. Increasing evidence has shown that stiffened matrix plays an important role in promoting CF activation and cardiac fibrosis, and several signaling factors mediating CF mechanotransduction have been identified. However, the key molecules that perceive matrix stiffness to regulate CF activation remain to be further explored. Here, we detected significantly increased expression and nuclear localization of Yes-associated protein (YAP) in native fibrotic cardiac tissues. By using mechanically regulated in vitro cell culture models, we found that a stiff matrix-induced high expression and nuclear localization of YAP in CFs, accompanied by enhanced cell activation. We also demonstrated that YAP knockdown decreased fibrogenic response of CFs and that YAP overexpression promoted CF activation, indicating that YAP plays an important role in mediating matrix stiffness-induced CF activation. Further mechanistic studies revealed that the YAP pathway is an important signaling branch downstream of angiotensin II type 1 receptor in CF mechanotransduction. The findings help elucidate the mechanism of fibrotic mechanotransduction and may contribute to the development of new approaches for treating fibrotic diseases.

Endothelial Forkhead Box Transcription Factor P1 Regulates Pathological Cardiac Remodeling Through Transforming Growth Factor-β1-Endothelin-1 Signal Pathway

BACKGROUND

Pathological cardiac fibrosis and hypertrophy, the common features of left ventricular remodeling, often progress to heart failure. Forkhead box transcription factor P1 (Foxp1) in endothelial cells (ECs) has been shown to play an important role in heart development. However, the effect of EC-Foxp1 on pathological cardiac remodeling has not been well clarified. This study aims to determine the role of EC-Foxp1 in pathological cardiac remodeling and the underlying mechanisms.

METHODS

Foxp1 EC-specific loss-of-function and gain-of-function mice were generated, and an angiotensin II infusion or a transverse aortic constriction operation mouse model was used to study the cardiac remodeling mechanisms. Foxp1 downstream target gene transforming growth factor-β1 (TGF-β1) was confirmed by chromatin immunoprecipitation and luciferase assays. Finally, the effects of TGF-β1 blockade on EC-Foxp1 deletion-mediated profibrotic and prohypertrophic phenotypic changes were further confirmed by pharmacological inhibition, more specifically by RGD-peptide magnetic nanoparticle target delivery of TGF-β1-siRNA to ECs.

RESULTS

Foxp1 expression is significantly downregulated in cardiac ECs during angiotensin II-induced cardiac remodeling. EC-Foxp1 deletion results in severe cardiac remodeling, including more cardiac fibrosis with myofibroblast formation and extracellular matrix protein production, as well as decompensated cardiac hypertrophy and further exacerbation of cardiac dysfunction on angiotensin II infusion or transverse aortic constriction operation. In contrast, EC-Foxp1 gain of function protects against pathological cardiac remodeling and improves cardiac dysfunction. TGF-β1 signals are identified as Foxp1 direct target genes, and EC-Foxp1 deletion upregulates TGF-β1 signals to promote myofibroblast formation through fibroblast proliferation and transformation, resulting in severe cardiac fibrosis. Moreover, EC-Foxp1 deletion enhances TGF-β1-promoted endothelin-1 expression, which significantly increases cardiomyocyte size and reactivates cardiac fetal genes, leading to pathological cardiac hypertrophy. Correspondingly, these EC-Foxp1 deletion-mediated profibrotic and prohypertrophic phenotypic changes and cardiac dysfunction are normalized by the blockade of TGF-β1 signals through pharmacological inhibition and RGD-peptide magnetic nanoparticle target delivery of TGF-β1-siRNA to ECs.

CONCLUSIONS

EC-Foxp1 regulates the TGF-β1-endothelin-1 pathway to control pathological cardiac fibrosis and hypertrophy, resulting in cardiac dysfunction. Therefore, targeting the EC-Foxp1-TGF-β1-endothelin-1 pathway might provide a future novel therapy for heart failure.

Unique mechanistic insights into the beneficial effects of angiotensin-(1-7) on the prevention of cardiac fibrosis: A metabolomic analysis of primary cardiac fibroblasts

BACKGROUND

Cell metabolic pathways are highly conserved among species and change rapidly in response to drug stimulation. Therefore, we explore the effects of angiotensin-(1-7) in a primary cell model of cardiac fibrosis established in angiotensin II-stimulated cardiac fibroblasts via metabolomics analysis and further clarify the potential protective mechanism of angiotensin-(1-7).

METHODS AND RESULTS

After exposing cardiac fibroblasts to angiotensin II and/or angiotensin-(1-7), 172 metabolites in these cells were quantified and identified by gas chromatography-mass spectrometry. The data were subsequently analyzed by orthogonal partial least squares discriminant analysis to shortlist biochemically significant metabolites associated with the antifibrotic action of angiotensin-(1-7). Seven significant metabolites were identified: 10,13-dimethyltetradecanoic acid, arachidonic acid, aspartic acid, docosahexaenoic acid (DHA), glutathione, palmitelaidic acid, and pyroglutamic acid. By metabolic network analysis, we found that these metabolites were involved in six metabolic pathways, including arachidonic acid metabolism, leukotriene metabolism, and the γ-glutamyl cycle. Since these metabolic pathways are related to calcium balance and oxidative stress, we further verified that angiotensin-(1-7) suppressed the abnormal extracellular calcium influx and excessive accumulation of intracellular reactive oxygen species (ROS) in angiotensin II-stimulated cardiac fibroblasts. Furthermore, we found that angiotensin-(1-7) suppressed the abnormal calcium- and ROS-dependent activation of calcium/calmodulin-dependent protein kinase II delta (CaMKIIδ), the increased expression of CaMKIIδ-related proteins (NADPH oxidase 4 (Nox4), cellular communication network factor 2 (CTGF), and p-ERK1/2), and excessive collagen deposition in vitro and in vivo.

CONCLUSIONS

Angiotensin-(1-7) can ameliorate the angiotensin II-stimulated metabolic perturbations associated with cardiac fibroblast activation. These metabolic changes indicate that modulation of calcium- and ROS-dependent activation of CaMKIIδ mediates the activity of angiotensin-(1-7) against cardiac fibrosis. Moreover, pyroglutamic acid and arachidonic acid may be potential biomarkers for monitoring the antifibrotic action of angiotensin-(1-7).

Cardiac Fibrotic Remodeling on a Chip with Dynamic Mechanical Stimulation

Cardiac tissue is characterized by being dynamic and contractile, imparting the important role of biomechanical cues in the regulation of normal physiological activity or pathological remodeling. However, the dynamic mechanical tension ability also varies due to extracellular matrix remodeling in fibrosis, accompanied with the phenotypic transition from cardiac fibroblasts (CFs) to myofibroblasts. It is hypothesized that the dynamic mechanical tension ability regulates cardiac phenotypic transition within fibrosis in a strain-mediated manner. In this study, a microdevice that is able to simultaneously and accurately mimic the biomechanical properties of the cardiac physiological and pathological microenvironment is developed. The microdevice can apply cyclic compressions with gradient magnitudes (5-20%) and tunable frequency onto gelatin methacryloyl (GelMA) hydrogels laden with CFs, and also enables the integration of cytokines. The strain-response correlations between mechanical compression and CFs spreading, and proliferation and fibrotic phenotype remolding, are investigated. Results reveal that mechanical compression plays a crucial role in the CFs phenotypic transition, depending on the strain of mechanical load and myofibroblast maturity of CFs encapsulated in GelMA hydrogels. The results provide evidence regarding the strain-response correlation of mechanical stimulation in CFs phenotypic remodeling, which can be used to develop new preventive or therapeutic strategies for cardiac fibrosis.

Heart Failure With Improved Ejection Fraction: Is it Possible to Escape One's Past?

Among patients with heart failure with reduced ejection fraction, investigators have repeatedly identified a subgroup whose left ventricular ejection fraction and structural remodeling can improve to normal or nearly normal levels with or without medical therapy. This subgroup of patients with "heart failure with improved ejection fraction" has distinct clinical characteristics and a more favorable prognosis compared with patients who continue to have reduced ejection fraction. However, many of these patients also manifest clinical and biochemical signs of incomplete resolution of heart failure pathophysiology and remain at some risk of adverse outcomes, thus indicating that they may not have completely recovered. Although rigorous evidence on managing these patients is sparse, there are several reasons to recommend continuation of heart failure therapies, including device therapies, to prevent clinical deterioration. Notable exceptions to this recommendation may include patients who recover from peripartum cardiomyopathy, fulminant myocarditis, or stress cardiomyopathy, whose excellent long-term prognoses may imply true myocardial recovery. More research on these patients is needed to better understand the mechanisms that lead to improvement in ejection fraction and to guide their clinical management.

Hydraulic forces contribute to left ventricular diastolic filling

Myocardial active relaxation and restoring forces are known determinants of left ventricular (LV) diastolic function. We hypothesize the existence of an additional mechanism involved in LV filling, namely, a hydraulic force contributing to the longitudinal motion of the atrioventricular (AV) plane. A prerequisite for the presence of a net hydraulic force during diastole is that the atrial short-axis area (ASA) is smaller than the ventricular short-axis area (VSA). We aimed (a) to illustrate this mechanism in an analogous physical model, (b) to measure the ASA and VSA throughout the cardiac cycle in healthy volunteers using cardiovascular magnetic resonance imaging, and (c) to calculate the magnitude of the hydraulic force. The physical model illustrated that the anatomical difference between ASA and VSA provides the basis for generating a hydraulic force during diastole. In volunteers, VSA was greater than ASA during 75-100% of diastole. The hydraulic force was estimated to be 10-60% of the peak driving force of LV filling (1-3 N vs 5-10 N). Hydraulic forces are a consequence of left heart anatomy and aid LV diastolic filling. These findings suggest that the relationship between ASA and VSA, and the associated hydraulic force, should be considered when characterizing diastolic function and dysfunction.

Role of circulating factors in cardiac aging

Worldwide increase in life expectancy is a major contributor to the epidemic of chronic degenerative diseases. Aging, indeed, simultaneously affects multiple organ systems, and it has been hypothesized that systemic alterations in regulators of tissue physiology may regulate this process. Cardiac aging itself is a major risk factor for cardiovascular diseases and, because of the intimate relationship with the brain, may contribute to increase the risk of neurodegenerative disorders. Blood-borne factors may play a major role in this complex and still elusive process. A number of studies, mainly based on the revival of parabiosis, a surgical technique very popular during the 70s of the 20th century to study the effect of a shared circulation in two animals, have indeed shown the potential that humoral factors can control the aging process in different tissues. In this article we review the role of circulating factors in cardiovascular aging. A better understanding of these mechanisms may provide new insights in the aging process and provide novel therapeutic opportunities for chronic age-related disorders.

Transcriptional control of cardiac fibroblast plasticity

Cardiac fibroblasts help maintain the normal architecture of the healthy heart and are responsible for scar formation and the healing response to pathological insults. Various genetic, biomechanical, or humoral factors stimulate fibroblasts to become contractile smooth muscle-like cells called myofibroblasts that secrete large amounts of extracellular matrix. Unfortunately, unchecked myofibroblast activation in heart disease leads to pathological fibrosis, which is a major risk factor for the development of cardiac arrhythmias and heart failure. A better understanding of the molecular mechanisms that control fibroblast plasticity and myofibroblast activation is essential to develop novel strategies to specifically target pathological cardiac fibrosis without disrupting the adaptive healing response. This review highlights the major transcriptional mediators of fibroblast origin and function in development and disease. The contribution of the fetal epicardial gene program will be discussed in the context of fibroblast origin in development and following injury, primarily focusing on Tcf21 and C/EBP. We will also highlight the major transcriptional regulatory axes that control fibroblast plasticity in the adult heart, including transforming growth factor β (TGFβ)/Smad signaling, the Rho/myocardin-related transcription factor (MRTF)/serum response factor (SRF) axis, and Calcineurin/transient receptor potential channel (TRP)/nuclear factor of activated T-Cell (NFAT) signaling. Finally, we will discuss recent strategies to divert the fibroblast transcriptional program in an effort to promote cardiomyocyte regeneration. This article is a part of a Special Issue entitled "Fibrosis and Myocardial Remodeling".

Mechanical control of cardiac myofibroblasts

Fibroblasts produce and turn over collagenous extracellular matrix as part of the normal adaptive response to increased mechanical load in the heart, e.g. during prolonged exercise. However, chronic overload as a consequence of hypertension or myocardial injury trigger a repair program that culminates in the formation of myofibroblasts. Myofibroblasts are opportunistically activated from various precursor cells that all acquire a phenotype promoting excessive collagen secretion and contraction of the neo-matrix into stiff scar tissue. Stiff fibrotic tissue reduces heart distensibility, impedes pumping and valve function, contributes to diastolic and systolic dysfunction, and affects myocardial electrical transmission, potentially leading to arrhythmia and heart failure. Here, we discuss how mechanical factors, such as matrix stiffness and strain, are feeding back and cooperate with cytokine signals to drive myofibroblast activation. We elaborate on the importance of considering the mechanical boundary conditions in the heart to generate better cell culture models for mechanistic studies of cardiac fibroblast function. Elements of the force transmission and mechanoperception apparatus acting in myofibroblasts are presented as potential therapeutic targets to treat fibrosis.